The present invention relates to a glow plug for use in a diesel engine.
Hereinafter, the term xe2x80x9cfrontxe2x80x9d refers to a heating end side with respect to the axial direction of a glow plug, and the term xe2x80x9crearxe2x80x9d refers to a side opposite the front side.
A glow plug is widely used for the preheating of a diesel engine, which comprises a metallic sleeve, a rod-shaped ceramic heater disposed in the metallic sleeve with a front end portion thereof protruded from the metallic sleeve and a metallic shell fitted onto the metallic sleeve by e.g. brazing. The ceramic heater generally includes an insulating ceramic substrate, a heating resistor embedded in a front end portion of the ceramic substrate and a pair of electric conductors (such as high-melting lead wires made of e.g. conductive ceramic or tungsten) embedded in the ceramic substrate and electrically connected to the heating resistor. In order to supply power to the heating resistor through the electric conductors, the electric conductors are exposed to the outside of the ceramic heater. More specifically, one of the electric conductors is exposed at a rear end surface of the ceramic heater and connected to power source (such as a battery) via a terminal member, and the other of the electric conductors is exposed at an outer circumferential surface of the ceramic heater and joined to the metallic sleeve so as to establish a ground, as disclosed in Japanese Laid-Open Patent Publication No. 4-268112.
In the above-mentioned structure, however, it is difficult to establish a proper joint for electrical connection between the metallic sleeve and the grounding conductor by welding or brazing while securing a large joint surface therebetween. If the joint is improper, the ceramic heater cannot be energized to generate heat sufficiently. In addition, there arises a possibility of undesired heat generation at the joint.
It is therefore an object of the present invention to provide a glow plug in which a proper electrical connection can be easily and assuredly established between the metallic sleeve and the grounding conductor of the ceramic heater.
According to one aspect of the present invention, there is provided a glow plug comprising: a ceramic heater having an insulating ceramic substrate, a heating resistor embedded in a front end portion of the ceramic substrate, and a pair of first and second electric conductors embedded in the ceramic substrate and electrically connected at front end portions thereof to the heating resistor; and a metallic sleeve circumferentially surrounding the ceramic heater with a front end portion of the ceramic heater protruded from the metallic sleeve, the first electric conductor having a rear end portion exposed at a rear end surface of the ceramic heater and electrically connected to the metallic sleeve.
According to another aspect of the present invention, there is provided a glow plug comprising: a ceramic heater having an insulating ceramic substrate, a heating resistor embedded in a front end portion of the ceramic substrate, and a pair of first and second electric conductors embedded in the ceramic substrate and electrically connected at front end portions thereof to the heating resistor; a metallic sleeve circumferentially surrounding the ceramic heater with a front end portion of the ceramic heater protruded from the metallic sleeve; a metallic shell fitted onto the metallic sleeve; and a central electrode disposed in a rear portion of the metallic shell, the first and second electric conductors having rear end portions exposed at a rear end surface of the ceramic heater and electrically connected to the metallic sleeve and the central electrode, respectively.